Decorated Great helms and Zweihander’s Helmets should be famed armour pieces with varying stats.
Think about it. These are supposed to be the best of the best equipment from the Noble Houses, worn by their elite Knights and Zweihanders. One would assume, thus, that their equipment wasn’t just crafted by some random town blacksmith, but by a renowned team of excellent craftsmen who make stuff for the ruling nobility and their bodyguards. As such, the stat bonuses on fatigue and durability make sense.
Making these two helmet types into famed items would allow for several things. First, reviving content. It would be awesome to find, for instance, the Castle themed Decorated Great Helm (cut from the game when the Castle Noble House was removed) being used by a Champion Brigand Leader in battle, who also uses the upside down Castle banner for his Brigands sub-faction.
Second, map retention. I know a lot of players, myself included, that will ditch amazing map seeds because they do not feature their favourite Noble Houses, and thus block access to our favourite Decorated Great Helms and Zweihander’s Helmets. By making these into famed items, we’d be able to acquire them either as loot, or buying them from town vendors.
And third, variation. Since map seeds only allow for the three existing Noble House helmets to be featured, by making these into famed items we’d get the chance to acquire the other variants of these helmets on any map. Yes, the grind would be much longer and tougher, but it leaves a lot more room for it to happen.
With these reasons in mind, and the veritable lack of downsides, I really see no issue in making this small, yet for some very significant, change.